Skid steer vehicles such as skid steer loaders are a mainstay of construction work. In their most common configuration, they have two drive wheels on each side of a chassis that are driven in rotation by one or more hydraulic motors coupled to the wheels on one side and another one or more hydraulic motors coupled to the wheels on the other side.
The wheels on one side of the vehicle can be driven independently of the wheels on the other side of the vehicle. This permits the wheels on opposing sides of the vehicle to be rotated at different speeds, in opposite directions, or both. By rotating in opposite directions, the skid steer can rotate in place about a vertical axis that extends through the vehicle itself.
The vehicles have an overall size of about 10 by 12 feet which, when combined with their ability to rotate in place, gives them considerable mobility at a worksite. It is this mobility that makes them a preferred vehicle.
Skid steer vehicles commonly have at least one loader lift arm that is pivotally coupled to the chassis of the vehicle to raise and lower at the operator's command. This arm typically has a bucket, blade or other implement attached to the end of the arm that is lifted and lowered thereby. Perhaps most commonly, a bucket is attached to the arm and the skid steer vehicle. This bucket is commonly used to carry supplies or particulate matter such as gravel, sand, or dirt around a worksite.
Skid steering provides its own problems. First, skidding tears the terrain over which the vehicle travels. The tighter the vehicle skid steers, the more damage to the ground it causes. A skid steer vehicle that turns in place can actually dig ruts into loose soil. Skid steer vehicles operated in close quarters rapidly denude soil by literally scrubbing the ground clean of grass and other plant matter.
The problems with skid steering have limited the use of skid steer vehicles to construction sites, road construction sites and other locations where there is no topsoil or plant matter to damage.
To answer the need for less ground-damaging vehicles, skid steer vehicles have been devised that provide limited steering of the drive wheels with respect to the chassis.
These skid steer vehicles have four wheels pivotable with respect to the chassis by four hydraulic cylinders. The cylinders are hydraulically coupled such that a single hydraulic valve is capable of simultaneously steering all four suspensions at once. When hydraulic fluid flows in a first direction, the front wheels are steered to the left and the rear wheels are steered to the right. When hydraulic fluid flows in the opposite direction, the front wheels are steered to the right and the rear wheels are steered to the left.
There are some limitations to this arrangement, however.
First, four hydraulic actuators are required, one for each wheel. Each wheel has its own separate actuator. It is a limitation to require four separate devices to turn the vehicle. The force levels required to turn the vehicle do not inherently require four actuators, however, and it would be beneficial to reduce their number.
Second, they do not permit the wheels to be steered independently of each other. They route fluid flow to all four steering cylinders through a single hydraulic valve. This valve simultaneously sends hydraulic fluid into all four steering actuators and in the same amounts. As a result, all the wheels turn together and turn the same amount. This arrangement does not permit the wheels to be steered independently or even semi-independently. As a result, the turning radii of the four wheels will not intersect at a common point. Whenever the turning radii of the wheels do not intersect at a common point, there will be tire scrubbing during the turn.
They only provide turning in place by skid steering. Their wheels cannot be oriented to reduce or eliminate wheel skidding when the vehicle is turned or rotated in place. The ability to turn in place is one of the most valued characteristics of the skid steer vehicle, hence the ability to reduce wheel skidding and tearing the ground while turning in place is of significant value.
What is needed, therefore, is an improved system of steering the wheels of a skid steer vehicle. What is also needed is a skid steer vehicle having an improved system of steering. What is also needed is a system for steering a skid steer vehicle that reduces the degree to which the wheels skid when the vehicle is turned. What is also needed is a system that permits the vehicle to be turned in place with reduced skidding. It is an object of this invention to provide one or more of these advantages.